1. Field of the Invention
The present invention relates to an anode material including at least one of silicon (Si) and tin (Sn) as an element, and an anode and a battery using the anode material.
2. Description of the Related Art
In recent years, a large number of portable electronic devices such as camcorders, digital still cameras, cellular phones, personal digital assistants and laptop computers have been emerged, and an attempt to reduce the size and the weight of them has been made. Accordingly, the development of lightweight secondary batteries capable of obtaining a higher energy density has been promoted. Among the batteries, a lithium-ion secondary battery using a carbon material for an anode, a composite material of lithium and a transition metal for a cathode and a carbonate for an electrolytic solution can obtain a high energy density, compared to a lead-acid battery and a nickel cadmium battery in related arts, so the lithium-ion secondary battery has been widely put into practical use.
Moreover, recently a further improvement in capacity of batteries has been desired according to the enhancement of performance of portable electronic devices, and the use of tin or silicon as an anode active material instead of the carbon material has been studied (for example, refer to U.S. Pat. No. 4,950,566). It is because the theoretical capacities of tin and silicon are 994 mAh/g and 4199 mAh/g, respectively, which are much larger than the theoretical capacity of graphite of 372 mAh/g, therefore, an improvement in capacity can be expected.
However, a tin alloy or a silicon alloy including lithium (Li) has high activity, so an electrolyte solution is easily decomposed, and lithium is inactivated. Therefore, when charge and discharge are repeated, charge-discharge efficiency declines, thereby it is difficult to obtain sufficient cycle characteristics.
Accordingly, techniques to form a film made of an oxide of silicon, titanium (Ti), aluminum (Al) or zirconium (Zr) on a surface of an anode active material by a sol-gel method have been proposed (for example, refer to Japanese Unexamined Patent Application Publication Nos. 2004-335334 and 2004-335335).